I have a 1950's something rambler. The walls are plaster which I love, but in the bathroom the walls have a grey discoloration to them that looks like water dripping down them. I have tried to bleach them, but that does not remove it. Do I replaster or tear down the walls or can I use kilz and paint over them without the stuff coming back? There is no ventilation in this area and I will put something in. I guess I wonder if it is mold or just wet plaster look. Thank you any advice would be helpful.